In the known art, as countermeasures utilized in connection with structures such as, for example, buildings as a means for enabling the structures to withstand earthquakes, there have been considered various designs for the buildings themselves so as to render the structures strong and thereby prevent the buildings from being destroyed or collapsing as a result of the earthquakes, and there have also been considered various designs for the buildings so as to render the structures flexible whereby the same would be capable of absorbing the energy of the earthquake which causes vibration of a building, particularly the floor thereof, with substantially the same amplitude of vibration as that of the earthquake.
In accordance with recent techniques for protecting a building from the effects of an earthquake, a soft and flexible material such as, for example, rubber has been incorporated within the foundation or the basic structure of the building so as to absorb the vibrations caused by the earthquake and to prevent the vibrations from being transmitted to, for example, located upon the base structure of the building, such as, for example, the floor of the building.
Although it may be possible to prevent the building from being destroyed or collapsing by designing current buildings as described above, the known art is not sufficiently effective so as to apply such teachings or technology to existing buildings or structures and, hence, has not adequately provided countermeasures against earthquakes so as to protect equipment such as, for example, precision instruments or office automation systems inclusive of electronic computers from being destroyed, damaged or colliding with each other as a result of such equipment being disposed within existing buildings or structures.
In addition, it is extremely troublesome to reconstruct an existing building so as to provide an isolation floor structure therewithin such that the building would be capable of withstanding the effects of an earthquake and, moreover, such reconstruction work involves considerable much cost.
In order to obviate the problems described above, there has also been proposed an isolation floor system for use within a building structure so as to accommodate the effects of an earthquake to which the oscillations or vibrations due to the earthquake cannot be transmitted when the earthquake occurs, and such isolation floor system is arranged within the building at locations particularly used for supporting computers, containers within which medicines or chemicals are accommodated, or emergency generators.
The known art further provides a countermeasure by means of which a floor upon which the computers, medicine containers or the emergency generators are disposed is partially constructed as an isolation floor structure, such as, for example, disclosed in the Japanese Patent Publication No. 60-59381 or Japanese Patent Laid-open Publication No. 59-47543.
With the isolation floor system of the disclosed type, an isolation floor is constructed so as to be movable in either one of the horizontal or vertical directions or in both of these directions with respect to a floor for supporting the isolation floor, and a damping device or shock absorbing device such as, for example, a spring means is located between the isolation floor and the supporting floor so as to absorb the vibrations caused during the occurence of the earthquake. Concretely, an X-directional damping device is utilized in connection with the isolation floor so as to render the same movable in the X-direction by roller means. A Y-directional damping device is disposed, above the X-directional isolation floor, so as to render the isolation floor movable in the Y-direction normal to the X-direction. A vertical damping device is further disposed, above the Y-directional isolation floor, in connection with an isolation floor which is movable in the vertical or Z direction. Accordingly, a three dimensional isolation floor system is constructed as a single structural entity.
With the isolation floor system of the character described above, however, it is necessary to design the isolation floor system for accommodating an earthquake so that the displacement amount of the entire isolation floor system is sufficiently larger than any predicted oscillation or vibration amount which may be caused by the earthquake in order to prevent the system from colliding with the surrounding equipment or structure such as, for example, the supporting floors. The application of the large displacement can be achieved by elongating the stroke of the damping device, however, such results in the provision of extra large or wasted space below the isolation floor. Moreover, with an existing building, since only a small space exists below the isolation floor to be utilized for the location of the damping device having a large stroke, it is substantially impossible to locate the isolation floor system within the existing building or structure and it is also difficult to reconstruct the floor system so as to serve as an isolation floor system for accommodating the effects of an earthquake.
Generally, the vibration prevention effect with respect to the earthquake can be improved by elongating the natural frequency of the oscillating portion, and the natural frequency is in inverse proportion to 1/2 the square of the rigidity and in proportion to 1/2 the square of the mass. Accordingly, in order to further increase the vibration prevention effect, it is advantageous to reduce the rigidity of the damping device such as, for example, the spring constant thereof in comparison with the mass of the oscillating portion. In order to satisfy this requirement, it is necessary to prepare a spring member having a long effective length, which requires a large space to locate the spring means, and more particularly, the spring means located below the vertically movable isolation floor is required to have a long length relative to the diameter of the spring means, which may result in the longitudinal buckling of the spring means.
There has also been proposed a damping means constructed by alternatingly laminating metallic plates and rubber members, such as, for example disclosed in "JAPANESE MECHANICAL INSTITUTE ASSOCIATION PAPERS, Vol. 53, No. 490". This discloses a damping means disposed normally to the vibration direction so as to absorb the vibration of the isolation floor due to the shearing resistance of the rubber members.
With the damping means of the type described above, the shearing resistance is made small as the axial load of the damping means becomes large, and there the earthquake vibration prevention effect is increased when a large load is applied such as in for example the case where the entire building structure is designed to be prevented from vibrating as a result of the earthquake. When the damping means of such character supports a relatively light isolation floor during an earthquake, the damping means is made elongated and, accordingly, buckling may be caused within the damping means. Moreover, according to the conventional isolation floor structure, it is difficult to adequately elongate natural frequencies, so that there is the fear of resonating along with the vibrations caused by means of the earthquake.